For thermodynamic reasons, steam turbines are used at relatively high temperatures. Recent development in modern turbomachine construction has tended toward planning for temperatures of over 700° C., and even over 720° C., in the inflow zone of a high-pressure turbine section. Such high temperatures lead to special thermal stresses on the materials used.
Steam turbines are conventionally divided into a plurality of turbine sections, e.g. a high-pressure, medium-pressure and low-pressure turbine section. The abovementioned turbine sections differ essentially in that the steam parameters, such as the temperature and pressure of the inflowing steam, are different. Thus, a high-pressure turbine section is exposed to the highest steam parameters and is thus subjected to the most severe thermal stress. The steam flowing out of the high-pressure turbine section is reheated by means of an intermediate superheater and flows into a medium-pressure turbine section, with the steam flowing into the low-pressure turbine section without intermediate superheating after flowing through the medium-pressure turbine section.
In general, the turbine sections are constructed separately, i.e. each turbine section comprises a separate casing. However, there are also known designs in which the high-pressure turbine section and the medium-pressure turbine section are accommodated in a common outer casing. Equally well known are turbine sections in which the medium-pressure component and the low-pressure component are arranged jointly in one outer casing.
Particularly in the high-pressure and the medium-pressure zone, the turbine sections are constructed with a rotor, an inner casing arranged around the rotor, and an outer casing. The rotor comprises rotor blades, which form a flow duct with the guide blades arranged in the inner casing. In general, the high-pressure turbine sections are of single-flow design, leading to a relatively high thrust due to the steam pressure on the rotor in one direction. The rotors are therefore generally constructed with dummy pistons. By admitting a flow to the dummy piston at a defined point, a pressure is produced, leading to a counterthrust which holds the rotor in the axial direction in a manner substantially free from forces.
The high temperatures require the use of materials which can withstand the high temperatures and pressures. Steels based on a nickel base or high-percentage chromium steels are also suitable for use at high temperatures.
In addition to the high temperatures, the components of a steam turbine must be of relatively corrosion-resistant design since many components are exposed to a flow of wet steam and, at the same time, the flow velocity of the steam is high. Given an encounter with wet steam in conjunction with a high flow velocity, such components would develop corrosion and erosion. This problem is currently eliminated by taking relatively expensive measures. One of said measures would be the use of high-chromium materials, for example, or the use of coatings which are applied to the components and thus avoid corrosion and erosion.
Particularly in the case of high-pressure turbine sections, the steam flowing out of the flow duct, which is essentially a wet steam, i.e. small water particles have formed in the steam, flows on components in the steam turbine, leading to damage, e.g. corrosion or erosion of the component. One known way of keeping this wet steam away from the components is to use protective shields.